In Japanese Pat. Publications Laid-open Nos. 116524/1985 and 116525/1985, there has already been disclosed a power transmission apparatus in which front wheels (or rear wheels) are directly driven by an engine, and a drive force is transmitted to rear wheels (or front wheels) through a coupling comprising a hydraulic oil pump. In the disclosed power transmission apparatus, when a difference in rotation speed occurs between a rotary shaft transmitting the drive force to the front wheels and that transmitting the drive force to the rear wheels, a drive force corresponding to the rotation speed difference is transmitted to the rear wheels by a static oil pressure of the hydraulic oil pump, thereby automatically obtaining a four-wheel drive condition in which the front and rear wheels are driven.
In such a power transmission apparatus, pressure of hydraulic oil delivered from the hydraulic oil pump is controlled to obtain a drive force transmitted to the rear wheels (or front wheels) matching the running conditions of the vehicle.
In the power transmission apparatus of a type in which the drive force is transmitted by the static pressure of the hydraulic oil pump to the rear wheels, the drive force is transmitted to the rear wheels in dependence on a relative difference in rotation speed between the drive shaft transmitting the drive force to the front wheels and that transmitting the drive force to the rear wheels. Therefore, when the vehicle is running at a constant speed in which the drive force is directly transmitted from the engine to the front wheels, the rotary shaft for the rear wheels rotates at the same rotation speed as the rotary shaft for the front wheels, and a floating condition occurs in which no drive force is transmitted to the rear wheels. In such a case, if a change in rotation speed is transmitted to a front wheel drive system due to a change in torque of the engine or the like, there occurs a difference in rotation speed between the rotary shafts for the front and rear wheels, a change in torque according to the change in rotation speed of the front wheels drive system is transmitted through the hydraulic oil pump to a rear wheel drive system, and unusual noises such as a hammering noise associated with the change in torque can occur in the rear wheel drive system, which will considerably impair the durability of the rear wheel drive system.